1. Field of the Invention
The present invention relates to an optical apparatus, suitable for use as part of a lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
Photolithography (also called microlithography) is used for manufacturing of semiconductor devices. Photolithography uses electromagnetic radiation, such as ultraviolet (UV), deep UV or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to implement photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle circuit pattern onto the wafer.
Instead of a mask, in some lithographic apparatus, the patterning device can be a patterning array that comprises an array of individually controllable elements. Sometimes, the pattern can be changed more efficiently in a maskless system compared to a mask-based system. These types of apparatus are referred to as Optical Maskless Lithographic (OML) apparatus.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a subsequent exposure. In a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a subsequent exposure.
Some illumination systems include an array or diffractive scattering optical element positioned before the reticle. The scattering optical element produces a desired angular light distribution that is subsequently imaged or relayed to the reticle.
Some conventional optical systems including lithographic systems use illuminators that include Excimer lasers as a light source and rely on every point of a wafer seeing multiple pulses (˜30) to average out effects from laser pulse to pulse fluctuations in pointing and divergence. However, one type of such system is an Optical Maskless Lithography (OML) system that may be a single pulse printing tool. Therefore, there is no pulse to pulse averaging to reduce laser fluctuation effects. In addition, a relatively high OML Projection Optics magnification may make control of laser pointing and divergence fluctuation tighter by ˜10× over many conventional optical or lithographic tools. Absent a way within the laser to control pulse to pulse fluctuation in an OML system, it is difficult to control pulse to pulse beam pointing and divergence to various degrees. However, pointing or divergence change manifests itself as an asymmetric or changing pupil.